The present invention relates to a method and a mould for manufacturing pellets of hot-melt ink.
Certain types of inkjet printers employ a so-called hot-melt ink, i.e., a wax-like ink material that is solid at room temperature and has a melting point in the order of 100 or 120° C., for example. In the print head of the printer, the ink is heated above its melting point, so that droplets of liquid ink can be expelled through the nozzles of the print head. In order to obtain a high quality of the printed image, the viscosity and hence the temperature of the molten ink in the print head should be maintained essentially constant. However, since the ink is consumed in the course of the printing process, and the ink reservoir accommodating the liquid ink within the print head is only of a limited size, it is necessary to supply and melt solid ink while the printer is operating, and the latent heat required for melting the ink tends to decrease the temperature in the ink reservoir. For this reason, it is desirable that the amount of solid ink supplied to the ink reservoir can precisely be controlled and metered, and, to this end, it is advantageous that the ink is supplied in the form of pellets having a predetermined size and shape, e.g., in the form of small spherical pills.
Since the hot-melt ink is a thermoplastic material, the pellets having the desired shape and size can be manufactured by means of a moulding process similar to injection moulding processes known for manufacturing articles from thermoplastic resin. The moulding process should, however, be adapted to the specific properties of hot melt ink which are in certain respects different from those of thermoplastic synthetic resins. Since the amount of shrinkage, which the hot-melt ink experiences when it is solidified, is comparatively low, and since a certain shrinkage can be tolerated because the final appearance of moulded ink pellets is not critical, it is not necessary to apply high locking forces for keeping the mould closed during the moulding process. On the other hand, since the hot-melt ink has a relatively low melting point, it tends to solidify immediately when it comes into contact with the walls of the mould cavity. This effect and the fact that the surface of the ink pellet is somewhat tacky even when the temperature has dropped below the melting point, increases the tendency of the pellet to adhere to walls of the mould cavity. This makes it more difficult to reliably and reproducibly remove the pellet from the mould.
EP1236577 is related to a method and a mould for manufacturing pellets of hot-melt ink. In particular, a mould is disclosed comprising two dies defining a mould cavity. The upper die defines also the runner hole through which the ink is guided to the mould cavity. In case the mould cavity is overfilled, there is the risk that after solidification of the ink and removal of the ink pellet from the mould, the ink pellet has a protrusion originating from the overfilled ink in the runner hole. On the other hand, in case the mould cavity is not completely filled, there is the risk that after solidification of the ink and the accompanying shrinkage, a hole remains in the ink pellet. Although the exact form of the pellets is not that important, both larger holes in the pellets and protrusions on the pellets are undesirable as they may hamper the free flow of ink pellets in an ink pellet dosing system. To avoid such protrusions or holes, an option could be to carefully control the pouring process such as to avoid overfilling or incompletely filling of the mould cavity. However, in practice it has been observed that even if one has excellent control over the pouring process, still part of the manufactured pellets have large holes or protrusions because of, e.g., the size variations of the mould cavities and variations of the ink viscosity which is detrimental for the manufacturing yield. One could also subject the pellets provided with a protrusion to an extra processing treatment in an attempt to remove the protrusion. Besides the fact that this substantially increases costs, it is observed that a large part of such pellets are still out of specifications because the protrusion is only partly removed or because by the removal of the protrusion a large hole is generated in the pellet.